The herein disclosed invention finds applicability in the field of fly ash improvement; and more specifically improving the performance of fly ash when used as an additive in air-entrained concrete.
Coal combustion fly ash is a useful additive in concrete due to its pozzolanic propertyxe2x80x94i.e., its ability to react with calcium in concrete mixes and thus contribute to the formation of the cementitious matrix. Through this mechanism, fly ash serves as a partial replacement for Portland cement, yielding cost savings as well as a variety of concrete property enhancements, which may include reduced permeability, improved workability, increased long-term strength, and reduced threat of long-term failure due to alkali-silica reaction.
A practical problem with this recycling technology is the tendency of residual carbon in ash is to interfere with the air entrainment process in concrete. Porous carbon adsorbs the chemical surfactants (air entraining admixtures, or AEAs) used to generate and stabilize a micro-void system in concrete pastes. Without a sufficient network of sub-millimeter air bubbles, concrete fails under internal pressure generated by the freezing and expansion of trapped residual water. About two-thirds of the concrete in North America is air entrained, and this surfactant adsorption phenomenon is the primary driving force for national and regional regulations limiting the carbon content of ash used in concrete.
Ash samples from the field show great variability in the extent to which they adsorb AEAs. Recent work has identified the following four primary factors governing ash adsorptivity: (1) the mass fraction carbon, (2) the total surface area of the carbon, (3) the accessibility of that surface, as governed by particle size and pore size distribution, and (4) the carbon surface chemistry. The inorganic fraction of ash is found to play a very minor role in AEA adsorption.
The role of carbon surface chemistry is particularly apparent from the behavior of ash during thermal oxidation in air. Introduction of surface oxides by exposure to air at 350-450xc2x0 C. has been observed to significantly reduce subsequent AEA adsorption without consuming a measurable amount of carbon. In contrast, treatment in inert gas at temperatures sufficient to drive-off many pre-exisiting surface oxides (900xc2x0 C.) has been observed in increase adsorption. Commercial carbon blacks subjected to surface oxidation processes have also been observed to be less adsorptive than non-treated varieties. Both of these observations suggest that oxide-free carbon surfaces are the most active for adsorption of surfactants. The important role of a non-polar (oxide-free) surface is not surprising, as polar functionalities are already abundant in concrete pastes (on inorganic fly ash particles, cement particles, aggregate particles, and in the aqueous solution), whereas the only non-polar components are air bubbles and a portion of the carbon surface. It is likely that the non-polar portions of the carbon surfaces compete directly with the air bubbles for the non-polar portions of the surfactant molecules. This insight suggests that the deleterious effect of carbon could be suppressed by intentional oxidation of the largely non-polar carbon surfaces.
Possibilities for intentional surface oxidation include dry and wet chemical methods. Many wet oxidation agents have been used to surface treat other carbon materials, including HNO3, H2O2CH3COOH, and (NH3)2S2O8, but for the treatment of ash these wet processes would have practical disadvantages, including high drying costs, and potential problems with self-cementation or loss of pozzolanic activity. Dry oxidation in air requires temperatures above about 300xc2x0 C., and is not likely to offer advantages over commercial combustion-based processes, which remove the carbon altogether while operating at only modestly higher temperatures. For these reasons, this patent focuses on ozone, O3, as an oxidant capable of attacking carbon surfaces in ash in the dry state and at ambient temperature.
Attention is brought to the fact that Gao et al in Fuel, Vol. 80 (2001), pages 765 to 768 have published an article related to the ozonation of fly ash.
Prior art U.S. Pat. No. 6,136,089 to Hurt et al teaches a method for ozonating fly ash in order to deactivate carbon in fly ash. The method of the patent uses 500 ppm ozone in air at 0.9 lit/min. in a packed or fixed bed treatment. Treatment of fly ash in a pneumatic conveyor tube is taught. The patent also teaches the use of a fluidized bed to ozonate fly ash. Ozonation of fly ash while the fly ash is stored in a silo is also taught. However, what is not taught by the patent, are unobvious, favorable processing conditions for the economic ozonation of fly ash. The favorable processing conditions are attained by making certain that there are favorable contact conditions between the ozone and the fly ash to be treated. The herein disclosed invention is designed to produce a process wherein the fly ash is optimally treated with ozone. The process assures that oversaturation does not occur with its attendant waste of ozone. For example, data presented in the patent clearly shows that sustained upflow through a stationary bed leads to oversaturation of the bottom portion of the bed with the attendant excessive use of ozone and the unnecessary gasification of carbon. Large-scale utility application of the process of U.S. Pat. No. 6,136,089 involving deep beds, and fixed-bed contacting schemes will be uneconomic. For this reason, the ash bed must be continuously mixed mixed or transported by mechanical or aerodynamic means to prevent prolonged contact of any part of the bed with fresh ozone. Further, the amount of ozone relative to the fly ash in the herein disclosed invention is controlled; e.g., gm-ozone/kg-carbon. This is a basis for the herein disclosed invention. Embodiments of the invention are presented below.
An important object of this invention is to treat fly ash with ozone in an efficient manner.
A most significant factor to bear in mind is that the invention disclosed herein recognized for the first time the importance of the quantity: gm-ozone/kg-carbon.
A further object of this invention is to treat fly ash with ozone in an economic manner.
These and other objects of the present invention will become apparent from a reading of the following specification taken in conjunction with the enclosed drawings.
In its broadest aspect, the herein disclosed invention is directed to ozone-treated carbon-containing fly ash wherein the ozone has treated the fly ash in a homogenous fashion so as not to over or under ozonize the fly ash. The ozone treated fly ash can be homogenously contacted wherein no portion of ash experiences prolonged exposure to ozone resulting in consumption of greater than 300 gram-ozone/kg-carbon for class C ashes or greater than 100 gram-ozone/kg-carbon for class F ashes.
The patent envisions a method for the homogeneous ozonation of fly ash to make the fly ash into a useful additive for concrete which method comprises continuously mixing fly ash with ozone assuring that no part of the fly ash has prolonged contact with the ozone such that no part of the fly ash consumes greater than 300 gram-ozone/kg-carbon for class C ashes or greater than 100 gram-ozone/kg-carbon for class F ashes and assuring that oversaturation with ozone does not occur.
Also, envisioned by this invention is a method for homogenous ozonation of fly ash to render said fly ash useful as an additive to concrete comprising operating a contactor in a batch mode to contact a fly ash bed with ozone for two minutes and purging the fly ash bed for three minutes with ozone-free air, wherein the fly ash is constantly brought into contact with the ozone such that the contacting takes place to produce a fly ash in the desired range of 10-100 gm-ozone per kg carbon in ash for class F ashes or 30-300 gm-ozone per kg carbon in class C ashes. The process can take place in a contactor able to employ 64 lit/sec of 5% ozone in either oxygen or air for two min/batch to yield 1.5 gm-ozone/kg-ash with the batch time comprising two minutes of ozone contact time and three minutes of purging. The process can have the fly ash contacted with 0.2-5 gm-ozone/kg of ash while avoiding over exposure of the fly ash with ozone.
Viewed another way the herein disclosed invention involves a process in which carbon-containing fly ash is treated with ozone-containing gas in the amount 0.2-5 g-ozone/kg ash to improve the quality of ash for applications in concrete or related construction products. Carbon-containing fly ash is treated with ozone-containing gas in the amount 10-300 g-ozone-kg-carbon to improve the quality of ash for applications in concrete or related construction products. The carbon-containing fly ash is treated with ozone-containing gas in the amount 1-50 xcexcmoles-ozone/m2xe2x80x94total-carbon-surface area to improve the quality of ash. The process of the invention contacts ash uniformly with an ozone-containing gas in such a way as to provide no opportunity for overexposure of part of the ash. In the process the ash is contacted with ozone in a device incorporating mechanical agitation, such as a solids blender, screw conveyer or ash conditioner. Alternatively, the process involves ash contacted with ozone in a device incorporating a fluid bed, spouted bed, circulating fluid bed, transport reactor, or a pneumatic conveying system.